Rare earth cold accumulating material particles, and refrigerator, superconducting magnet, inspection device and cryopump using same

ABSTRACT

The present invention provides a rare earth cold accumulating material particle comprising a rare earth oxide or a rare earth oxysulfide, wherein the rare earth cold accumulating material particle is composed of a sintered body; an average crystal grain size of the sintered body is 0.5 to 5 μm; a porosity of the sintered body is 10 to 50 vol. %; and an average pore size of the sintered body is 0.3 to 3 μm. Further, it is preferable that the porosity of the rare earth cold accumulating material particle is 20 to 45 vol. %, and a maximum pore size of the rare earth cold accumulating material particle is 4 m or less. Due to this structure, there can be provided a rare earth cold accumulating material having a high refrigerating capacity and a high strength.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/509,334, filed Mar. 7, 2017, which in turn is the national stageentry of International Application No. PCT/JP2015/075184, filed Sep. 4,2015, which designated the United States, the entireties of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present embodiment relates to rare earth cold accumulating materialparticles, and a refrigerator, a superconducting magnet, an inspectiondevice and a cryopump using the same.

BACKGROUND OF THE INVENTION

Recently, the development of the superconducting technology has beenremarkable, and with the expansion of the application fields of thesuperconducting technology, the development of small-size andhigh-performance refrigerators has been essential. Examples of the typeof the refrigerator include various types such as a GM (Gifford-McMahon)type, a pulse type and a Stirling type. Examples of the products usingthese refrigerators include a superconducting magnet, MRI, NMR, acryopump, a superconducting magnetic energy storage (SMES) and a singlecrystal pulling-up apparatus in a magnetic field for producing siliconwafers and the like.

In such a refrigerator, a working medium such as compressed He gas flowsin one direction in a cold accumulator filled with a cold accumulatingmaterial, the thermal energy of the working medium is supplied to thecold accumulating material, and the expanded working medium flows in theopposite direction and receives the thermal energy from the coldaccumulating material. As the recuperative effect in such a processbecomes satisfactory, the thermal efficiency in the working medium cycleis improved, and it is made possible to actualize lower temperatures.

As such a cold accumulating material as described above filled in thecold accumulator of a refrigerator, Cu, Pb and the like have hithertobeen mainly used. However, such a cold accumulating material has aremarkably small specific heat at ultralow temperatures of 20K or lower,accordingly does not allow the above-described recuperative effect tosufficiently function, cannot store a sufficient thermal energy in thecold accumulating material every one cycle at an ultralow temperatureduring the action in a refrigerator, does not allow the working mediumto receive a sufficient thermal energy from the cold accumulatingmaterial.

Consequently, a refrigerator incorporating the cold accumulator filledwith the cold accumulating material suffers from a problem that such arefrigerator is not allowed to reach an ultralow temperature. Thus,nowadays, in order to improve the recuperation property at ultralowtemperatures of the cold accumulator and to actualize a refrigerationtemperature closer to the absolute zero temperature, in particular,there are used rare earth cold accumulating materials mainly composed ofintermetallic compounds including rare earth elements and transitionmetal elements such as Er₃Ni, ErNi and HoCu₂ each having a local maximumvalue of the volume specific heat in the ultralow temperature region of20K or lower wherein the local maximum value is large. By using such arare earth cold accumulating material in a GM refrigerator, therefrigeration at 4K is actualized.

Along with the developed investigation of the application of such arefrigerator to various systems, from the technical requirements forstably cooling larger-scale cooling objects, refrigerators are requiredto be further improved in refrigerating capacity. In order to meet therequirements, recently, an attempt has been made to improve therefrigerating capacity by replacing part of a conventional metal-basedmagnetic cold accumulating material with a rare earth element-containingrare earth oxysulfide such as Gd₂O₂S.

A rare earth oxysulfide has a peak of the specific heat at 5K or lower,which is lower than the peak temperature of the specific heat of a rareearth cold accumulating material. Accordingly, an improvement of therefrigerating capacity can be achieved by using a rare earth oxysulfideas laminated on a rare earth cold accumulating material mainly composedof an intermetallic compound, having a large volume specific heat in thetemperature region of 6K or higher. A rare earth oxide cold accumulatingmaterial such as GdAlO₃ has a low specific heat peak, and provides aneffect similar to the effect obtained from a rare earth oxysulfide coldaccumulating material.

Rare earth oxysulfide cold accumulating materials are disclosed inJapanese Patent No. 3642486 (Patent Document 1) and Japanese Patent No.4582994 (Patent Document 2). In Patent Document 1 and Patent Document 2,high density sintered bodies each having a relative density of 98% ormore are obtained by using a tumbling granulation method.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent No. 3642486-   Patent Document 2: Japanese Patent No. 4582994-   Patent Document 3: Japanese Patent Laid-Open No. 2004-75884

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In general, cold accumulating materials are used as processed intospherical particles of approximately 0.2 mm (200 μm) in particle size,in order to efficiently perform heat exchange with a working medium suchas He gas, or in order to enhance the charging efficiency (fillingefficiency) of the cold accumulating material into the cold accumulatorof the refrigerator. The processing into spherical shapes allows thestrength of the cold accumulating material to be increased.

The heat exchange with a working medium such as He gas is carried out atthe portions where the working medium and the cold accumulating materialare brought into contact with each other. A rare earth cold accumulatingmaterial composed of a high density sintered body does not sufficientlymake the most of the advantage of the contact of the working medium onlyto the surface portion of a cold accumulating material and the advantageof the low specific heat peak of the material of the rare earth coldaccumulating material.

In order to cope with such problems, Japanese Patent Laid-Open No.2004-75884 (Patent Document 3) discloses a rare earth oxysulfide coldaccumulating material particle composed of a porous material having arelative density of 60 to 85%. In other words, in Patent Document 3, byspecifying the relative density so as to fall within a predeterminedrange, a rare earth oxysulfide cold accumulating material particlehaving pores is actualized. By having such constitution as describedabove, the compatibility between the air permeability and the strengthis achieved. However, the pore sizes in the interior of the rare earthoxysulfide cold accumulating material particles are not controlled, andhence the upgrading of the improvement effect of the air permeability islimited.

Means for Solving the Problems

The rare earth cold accumulating material particles according to theembodiment are designed to solve the above-described problems, arecomposed of a rare earth oxide or a rare earth oxysulfide, wherein therare earth cold accumulating material particles are composed of asintered body, the average crystal grain size of the sintered body is0.5 to 5 μm, the porosity of the sintered body is 10 to 50 vol. %, andthe average pore size is 0.3 to 3 μm.

Advantages of the Invention

According to the rare earth cold accumulating material according to theembodiment, in the rare earth cold accumulating material including arare earth oxide or a rare earth oxysulfide, the rare earth coldaccumulating material is composed of a sintered body; the averagecrystal grain size, the porosity and the average pore size of thesintered body are controlled; accordingly, the working medium (He gas)is brought into contact with the interior of the pores; consequently,the interior of the sintered body can also be used as a heat exchangemember. Thus, the refrigerating capacity can be drastically improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique perspective view illustrating an example of therare earth cold accumulating material particle according to anembodiment.

FIG. 2 is a plan view illustrating an example of an arbitrary crosssectional structure of the rare earth cold accumulating materialparticle according to the embodiment.

FIG. 3A and FIG. 3B are plan views each illustrating an example of thestructure of connected pores.

FIG. 4 is an oblique perspective view illustrating a group of the rareearth cold accumulating material particles according to an embodiment.

FIG. 5 is a cross sectional view illustrating an example of the secondcooling stage of the refrigerator according to an embodiment.

FIG. 6 is a cross sectional view illustrating another example of thesecond cooling stage of the refrigerator according to the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The rare earth cold accumulating material particle of an embodiment is arare earth cold accumulating material particle including a rare earthoxide or a rare earth oxysulfide, wherein the rare earth coldaccumulating material particle is composed of a sintered body; theaverage crystal grain size of the sintered body is 0.5 to 5 μm; theporosity of the sintered body is 10 to 50 vol. %; and the average poresize of the sintered body is 0.3 to 3 μm.

FIG. 1 shows an example of the rare earth cold accumulating materialparticle. In FIG. 1, the reference numeral 1 denotes the rare earth coldaccumulating material particle. The rare earth cold accumulatingmaterial particle has a spherical shape having an aspect ratio ofpreferably 2 or less and more preferably 1.5 or less.

FIG. 2 also shows an example of an arbitrary cross section of the rareearth cold accumulating material particle. In FIG. 2, the referencenumeral 2 denotes a rare earth compound crystal grain, the referencenumeral 3 denotes a pore and the reference numeral 4 denotes a structureof connected pores. The rare earth compound crystal grain is a rareearth oxide or a rare earth oxysulfide. FIG. 4 is an oblique perspectiveview illustrating a group of the rare earth cold accumulating materialparticles 1.

Examples of the rare earth oxide may include composite oxides such asrare earth aluminum oxides. The rare earth oxide is preferably agadolinium aluminum oxide, in particular, GdAlO₃. The rare earthoxysulfide is preferably a gadolinium oxysulfide, in particular, Gd₂O₂S.If necessary, a sintering aid may also be added to the rare earth oxideor the rare earth oxysulfide.

The rare earth cold accumulating material particle including the rareearth oxide or the rare earth oxysulfide is constituted with a sinteredbody. Here, the sintered body means a body obtained by molding astarting material powder, and by heating and sintering the resultingmolded body. The starting material powder is a main starting materialpowder composed of a rare earth oxide or a rare earth oxysulfide andincluding, if necessary, a sintering aid powder as mixed in the mainstarting material powder. It is difficult to prepare a rare earth coldaccumulating material particle composed of a rare earth oxide or a rareearth oxysulfide by melting the starting material although the rareearth cold accumulating material particle mainly composed of anintermetallic compound such as HoCu₂ can be prepared by melting thestarting material. Accordingly, it is effective to prepare a sinteredbody by heating.

In the sintered body constituting the particle, the average crystalgrain size is 0.5 to 5 μm. When the average crystal grain size is lessthan 0.5 μm, the crystal grains are too small, and accordingly it isdifficult to control the porosity. On the other hand, when the averagecrystal grain size exceeds 5 μm to be too large, the strength of therare earth cold accumulating material is degraded.

The measurement method of the average crystal grain size is as follows.Specifically, in an arbitrary cross section of the rare earth coldaccumulating material particle, an enlarged photograph of a unit area of10 μm×10 μm is taken. The enlarged photograph is a SEM photograph of2000 or more in magnification. In each of the crystal grains of the rareearth oxide or the rare earth oxysulfide, shown in the enlargedphotograph, the longest diagonal is measured as a major axis. Theaverage value of the major axes of 100 of the crystal grains is taken asthe average crystal grain size.

The porosity of the rare earth cold accumulating material particle isspecified to fall within a range from 10 to 50 vol. %. When the porosityis less than 10 vol. %, the effect of the provision of the pores is notsufficient. On the other hand, when the porosity exceeds 50 vol. % to betoo large, the strength of the cold accumulating material particle isdegraded. The porosity set to fall within a range from 10 to 50 vol. %allows the interior of the rare earth cold accumulating materialparticle composed of a sintered body to be brought into contact with theworking medium (He gas), and thus, the cold accumulating effect isimproved. The porosity is preferably set to 20 to 45 vol. %.

The average pore size (average pore diameter) is preferably set to 0.3to 3 μm. When the average pore size is too small as less than 0.3 μm, itis difficult for the cooling medium gas (He gas) to enter the interiorof the rare earth cold accumulating material particle. On the otherhand, when the average pore size exceeds 3 μm to be too large, thestrength of the cold accumulating material particle is degraded.

The maximum pore size (maximum pore diameter) is preferably 4 μm orless. When the maximum pore size exceeds 4 μm to be too large, thestrength of the cold accumulating material particle is liable to bedegraded. Accordingly, the maximum pore size is preferably 4 μm or lessand further preferably 2 μm or less. The minimum value of the maximumpore size is not particularly limited, but is preferably 0.5 μm or more.In other words, the maximum pore size preferably falls within a rangefrom 0.5 to 4 μm.

In an arbitrary cross section of the rare earth cold accumulatingmaterial particle, the number of the pores per the unit area of 10 μm×10μm is preferably 20 to 70. When the number of the pores per the unitarea is too small as less than 20, the cold accumulating effect issmall. On the other hand, when the number of the pores per the unit areais too large as exceeding 70, the strength of the cold accumulatingmaterial particle is liable to be degraded. In particular, the number ofthe pores having the pore sizes equal to or less than the maximum poresize of 4 μm is preferably limited to 20 to 70 per the unit area of 10μm×10 μm, and further preferably falls within a range from 30 to 60.

In an arbitrary cross section of the rare earth cold accumulatingmaterial particle, part of the pores present in the unit area of 10μm×10 μm preferably have structures of connected pores. FIG. 2 shows astructure 4 of connected pores as an example. In an arbitrary crosssection, the micro region of the unit area of 10 μm×10 μm has structuresof connected pores, and hence the air permeability of the interior ofthe rare earth cold accumulating material particle can be improved.

FIG. 3 shows examples of the structure of connected pores. FIG. 3A showsa structure of two connected pores, whereas FIG. 3B shows an example ofa structure of three connected pores. The rare earth cold accumulatingmaterial particle according to an embodiment is not limited to such astructure, and may have a structure of four or more connected pores. Asshown in FIG. 3, the structure of connected pores gives a shape formedby connected circles (inclusive of ellipses).

The measurement methods of the average pore size, the maximum pore size,the number of the pores, and the structure of the connected pores are asfollows. First, in an arbitrary cross section of the rare earth coldaccumulating material particle, an enlarged photograph of the unit areaof 10 μm×10 μm is taken. The enlarged photograph is a SEM photograph of2000 or more in magnification.

For easy observation of the pores, secondary electron images are used.In the secondary electron images of the SEM photographs, the pores areshown in black. The longest diagonal of the pore observed in thesecondary electron image is taken as the maximum diameter of the pore.The average value of the maximum diameters of the pores shown in theunit area of 10 μm×10 μm is obtained. The number of the pores shown inthe unit area of 10 μm×10 μm is counted. Here, in the portions havingthe structures of connected pores, each structure of connected pores iscounted as one pore. This operation is performed in five of the unitareas (one unit area: 10 μm×10 μm). In these five unit areas, thelongest diagonal is taken as the maximum pore size. The average poresize and the average number of the pores are determined as the averagevalues over the five unit areas, respectively.

By controlling, as described above, the average crystal grain size, thevolume proportion of the pores, the average pore size, and the maximumpore size, the cold accumulating effect can be sufficiently improvedwhile the mechanical strength of the rare earth cold accumulatingmaterial particle composed of a sintered body is being maintained. Inparticular, the number and the sizes of the pores are controlled in themicro region (unit area: 10 μm×10 μm) in the interior of the rare earthcold accumulating material particle, and hence the cold accumulatingproperty can be improved.

The rare earth cold accumulating material particles preferably have anaverage particle size of 100 to 500 μm. The average particle size set tofall within a range from 100 to 500 μm allows the filling factor of thecold accumulating material particles in the cooling stage of arefrigerator to be improved so as to fall within a range from 55 to 70%.In order to improve the filling factor, the average particle size of theparticles is preferably specified to be 150 to 300 μm.

When L represents the perimeter length of the projection image of eachof the cold accumulating material particles constituting a group of rareearth cold accumulating material particles, and A represents the actualarea of the projection image, it is preferable that in the group of rareearth cold accumulating material particles, the proportion of the rareearth cold accumulating material particles having the shape factor Rbeing represented by L²/4 πA and exceeding 1.5 be preferably 5% or less.In other words, by allowing each of the rare earth cold accumulatingmaterial particles to have a shape substantially close to a sphere, thefilling factor can be improved, and a path (passage) of the workingmedium gas (He gas) can be formed in the mutual gaps between the rareearth cold accumulating material particles.

The rare earth cold accumulating material particles according to anembodiment are effective for a refrigerator. In particular, theabove-described particles are effective for a refrigerator for obtainingan ultralow temperature region of 10K or lower. The rare earth coldaccumulating material particles are filled in a cold accumulating vesselof the refrigerator. When the rare earth cold accumulating materialparticles are filled in the cold accumulating vessel, a group of therare earth cold accumulating material particles including a large numberof the rare earth cold accumulating material particles according to anembodiment is prepared. The group of the rare earth cold accumulatingmaterial particles preferably includes the rare earth cold accumulatingmaterial particles according to an embodiment in a content of 50% bymass or more and 100% by mass or less.

FIG. 5 and FIG. 6 each show an example of the use of the group of therare earth cold accumulating material particles as filled in the coldaccumulating vessel of a refrigerator. In FIG. 5 and FIG. 6, thereference numeral 1-1 denotes a first group of cold accumulatingmaterial particles, the reference numeral 1-2 denotes a second group ofcold accumulating material particles, the reference numeral 1-3 denotesa third group of cold accumulating material particles, the referencenumeral 5 denotes a cold accumulating vessel, and the reference numeral6 denotes a metal mesh.

As the refrigerator, there are various types such as a GM-typerefrigerator, a Sterling-type refrigerator and a pulse-typerefrigerator. In the case of any one of these, an ultralow temperatureof 10K or lower, and furthermore an ultralow temperature of 4K or lowercan be achieved. In order to obtain an ultralow temperature, it isnecessary to fill a cold accumulating material in each of the coldaccumulating vessels referred to as a first cooling stage and a secondcooling state, respectively. If necessary, a third cooling stage mayalso be installed.

In FIG. 5, the interior of the second cooling stage of the refrigeratingvessel (cold accumulating vessel) is divided into two filling layers. InFIG. 6, the interior of the second cooling stage is divided into threefilling layers. In each of the regions, a group of cold accumulatingmaterial particles is filled. Metal meshes are disposed above and beloweach group of cold accumulating material particles, and the metal mesheshold the respective groups of cold accumulating material particles whilemaintaining air permeability. The groups of cold accumulating materialparticles are used so as for the specific heat peak of the coldaccumulating material particles to become lower on going from the firstcooling stage, through the second cooling stage, to the third coolingstage.

As the metal mesh 6, a copper (Cu) mesh is preferable. The specific heatof copper is low, and hence the copper mesh has an effect as a coldaccumulating material. Copper meshes may be used as a plurality ofcopper meshes superposed on each other. The mesh size is set at a sizenot allowing the group of cold accumulating material particles to passtherethrough.

FIG. 5 shows a two-layer type in which the interior of the secondcooling stage is provided with a plurality of filling layers through theintermediary of metal meshes, wherein the filling layer filling thefirst group of cold accumulating material particles 1-1 and the fillinglayer filling the second group of cold accumulating material particles1-2 are provided.

FIG. 6 shows a three-layer type in which the filling layer filled withthe first group of cold accumulating material particles 1-1, the fillinglayer filled with the second group of cold accumulating materialparticles 1-2 and the filling layer filled with the third group of coldaccumulating material particles 1-3 are provided. As a matter of course,a one-layer type or a four-layer type may also be adopted.

When the second cooling stage is divided into a plurality of fillinglayers, at least in one filling layer, a group of rare earth coldaccumulating material particles according to an embodiment is used. Forexample, in the case of the two-layer type, a combination is quoted inwhich a group of HoCu₂ particles is used for the first group of coldaccumulating material particles, and a group of rare earth coldaccumulating material particles (for example, a group of Gd₂O₂Sparticles) according to an embodiment is used for the second group ofcold accumulating material particles.

For example, in the case of the three-layer type, a combination isquoted in which a group of lead cold accumulating material particles isused as the first group of cold accumulating material particles, a groupof HoCu₂ particles is used as the second group of cold accumulatingmaterial particles, and a group of rare earth cold accumulating materialparticles (for example, a group of Gd₂O₂S particles) according to anembodiment is used as the third group of cold accumulating materialparticles.

In the combinations of the cold accumulating materials, the coldaccumulating material having a higher specific heat peak temperature isadopted as the first group of cold accumulating material particles, andthe cold accumulating material having a lower specific heat peaktemperature is adopted as the second group of cold accumulating materialparticles, and the groups of cold accumulating material particles arecombined in the decreasing order of the specific heat peak temperature.

When the interior of the cold accumulating vessel is partitioned withmetal meshes, it is preferable that groups of cold accumulating materialparticles be filled in the filling layers, respectively and pressed withthe metal meshes, and thus be filled so as for the gaps between themetal meshes and the groups of cold accumulating material particles tobe made as small as possible. The gaps between the metal meshes and thegroups of cold accumulating material particles have spaces, thevibration during the operation of the refrigerator or the pressure ofthe helium gas displaces the cold accumulating materials within thefilling layers, and the cold accumulating materials are liable to bedestructed.

Next, the method for producing the rare earth cold accumulating materialparticles according to an embodiment is described. The method forproducing the rare earth cold accumulating material particles accordingto an embodiment is not particularly limited as long as the rare earthcold accumulating material particles according to the embodiment havethe above-described constitution. However, as the method for efficientlyobtaining the rare earth cold accumulating material particles accordingto an embodiment, the following method is adopted.

First, a rare earth compound powder to be a starting material for therare earth cold accumulating material particles is prepared. Forexample, when a GdAlO₃ cold accumulating material is produced, a GdAlO₃powder is prepared. Alternatively, when a Gd₂O₂S cold accumulatingmaterial particle is produced, a Gd₂O₂S powder is prepared.

The rare earth compound powder to be used as the starting materialpreferably has an average particle size of 0.3 to 5 μm. When the averageparticular size is less than 0.3 μm, or exceeds 5 μm, it is difficult tocontrol the average crystal grain size of the sintered body so as to be0.5 to 5 μm. If necessary, a sintering aid powder may also be added. Thecontent (addition amount) of the sintering aid powder is set to be 1part by mass or more and 20 parts by mass or less in relation to 100parts by mass of the rare earth compound powder.

When the average particle size of the sintering aid powder isrepresented by A (μm), and the average particle size of the rare earthcompound powder is represented by B (μm), B/A is preferably regulated soas to fall within a range from 0.7 to 1.3. When the difference betweenthe average particle size of the rare earth compound powder and theaverage particle size of the sintering aid powder is excessively large,the control of the average pore size in the sintered body is difficult.

Next, the molding step is performed. In the molding step, preferable isa method using such a tumbling granulation step as described in theparagraph [0055] of Patent Document 2. When the tumbling granulationstep is performed, a resin binder is added. The addition amount of theresin binder is such that the resin binder is added so as for theaddition amount of the resin binder to be 10 to 50 vol. % in relation to100 vol. % of the total amount of the rare earth compound powder and theresin binder.

When the sintering aid powder is added, the sintering aid powder isadded so as for the addition amount of the resin binder to be 10 to 50vol. % in relation to 100 vol. % of the total amount of the rare earthcompound powder, the sintering aid powder and the resin binder.

The resin binder is dissipated in the sintering step. By setting theaddition amount of the resin binder to be 10 to 50 vol. %, the resinbinder dissipated in the sintering step gives rise to pores. The rareearth compound powder and the resin binder are mixed with each other ina predetermined volume ratio, and then the resulting mixture issufficiently stirred. After a starting material paste is prepared inwhich the rare earth compound powder and the resin binder are uniformlymixed with each other, the molding step is performed.

For the molding step, the methods such as tumbling granulation and diemolding are quoted. These methods may also be combined. From the moldingstep, a spherical molded body is obtained. The average particle size ofthe spherical molded body preferably falls within a range from 100 to500 μm.

Next, the sintering step is performed. The sintering step preferablyperforms a heat treatment at a temperature of 1200° C. or higher and2000° C. or lower. The sintering step dissipates the resin binder. Thesintering step can bind the rare earth compound powder particles to eachother. In order to increase the mutual binding force of the rare earthcompound powder particles, the sintering temperature is preferably setto be 1500° C. or higher, and the sintering time is preferably 1 hour ormore and 48 hours or less. The atmosphere for the sintering step may bea pressurized atmosphere.

When the sintering temperature exceeds 2000° C., or the heat treatmentis performed for a time as long as 48 hours or more, the crystal grainsof the rare earth compound (a rare earth oxide or a rare earthoxysulfide) undergo excessive grain growth, and the targeted averagecrystal grain size is liable not to be attained.

The sintering step yields a spherical sintered body. For the sphericalsintered body, if necessary, a surface polishing process is performed.

In the case of the rare earth cold accumulating material particlescomposed of a rare earth oxide sintered body, the heat treatment ispreferably performed in an oxygen atmosphere. In the case of the rareearth cold accumulating material particles composed of a rare earthoxysulfide sintered body, the heat treatment is preferably performed ina sulfur atmosphere including a sulfur oxide such as SO₂.

By the sintering step or the surface polishing process, the recovery ofthe portions lacking oxygen or sulfur can be performed. Consequently, asdisclosed in Patent Document 2, when irradiation with a light beamhaving a wavelength of 400 nm to 600 nm is applied, the reflectance ofthe surface portion of the cold accumulating material particles can bemade to be 30% or more and 95% or less. In Patent Document 2, the heattreatment temperature is described to be preferably 900 to 1200° C.

In the present embodiment, the surface polishing process and the heattreatment step may be performed in combination. If necessary, thespherical sintered body is to be subjected to a shape classification.The shape classification selects and adopts the spherical sintered bodyhaving an aspect ratio of preferably 2 or less and more preferably 1.5or less.

As the shape classification, when L represents the perimeter length ofthe projection image of each of the cold accumulating material particlesconstituting a group of rare earth cold accumulating material particles,and A represents the actual area of the projection image, it iseffective that in the group of rare earth cold accumulating materialparticles, the proportion of the rare earth cold accumulating materialparticles having the shape factor R being represented by L²/4πA andexceeding 1.5 is made to be 5% or less.

On the basis of the above-described production method, the rare earthcold accumulating material particles according to an embodiment can beefficiently obtained.

EXAMPLES Examples 1 to 6 and Comparative Examples 1 to 4

As a rare earth oxide, a gadolinium aluminum oxide (GdAlO₃) powderhaving an average particle size of 2 μm was prepared. In addition, as arare earth oxysulfide, a gadolinium oxysulfide (Gd₂O₂S) powder having anaverage particle size of 2 μm was prepared. Each of these powders wasmixed with a resin binder under the condition shown in Table 1.

The addition amount of the resin binder is given in a proportion inrelation to 100 vol. % of the total amount of the rare earth compoundpowder and the resin binder.

TABLE 1 Addition Amount of Rare Earth Resin Binder Sample No. CompoundPowder (vol %) Example 1 GdAlO₃ 23 Example 2 GdAlO₃ 31 Example 3 GdAlO₃43 Comparative GdAlO₃ 5 Example 1 Comparative GdAlO₃ 70 Example 2Example 4 Gd₂O₂S 22 Example 5 Gd₂O₂S 32 Example 6 Gd₂O₂S 42 ComparativeGd₂O₂S 5 Example 3 Comparative Gd₂O₂S 60 Example 4

A starting material paste was prepared by mixing the rare earth compoundpowder and the resin binder, and then the molding step was performed bythe tumbling granulation method. For the obtained spherical molded body,the sintering step was performed at 1850° C. for 2 hours. Subsequently,spherical sintered bodies having an aspect ratio of 1.5 or less werecollected by shape classification.

Further, a shape classification was performed in such a way that when Lrepresented the perimeter length of the projection image of each of thecold accumulating material particles constituting a group of rare earthcold accumulating material particles, and A represented the actual areaof the projection image, in the group of rare earth cold accumulatingmaterial particles, the proportion of the rare earth cold accumulatingmaterial particles having the shape factor R being represented by L²/4πAand exceeding 1.5 was 5% or less.

Through these steps, the rare earth cold accumulating material particlesaccording to Examples and Comparative Examples were produced. Theaverage particle size of the rare earth cold accumulating materialparticles in each of Examples and Comparative Examples was set to be 250μm.

For the rare earth cold accumulating material particles according toeach of Examples and Comparative Examples, the average crystal grainsize, the porosity, the average pore size, the maximum pore size and thenumber of the pores per unit area of 10 μm×10 μm were measured.

The measurements of these values were performed as follows. An enlargedphotograph (magnification: 3000) of an arbitrary cross section was takenby SEM. The major axes of the rare earth compound crystal grains shownin the unit area of 10 μm×10 μm were measured. The average value of themajor axes of 100 of the rare earth crystal grains was taken as theaverage crystal grain size.

For the porosity, the maximum pore size and the number of the pores, thearea proportion (%) of the pores, the largest pore size and the numberof the pores shown in the unit area of 10 μm×10 μm of the enlargedphotograph were determined.

This operation was performed in five of arbitrary unit areas (one unitarea: 10 μm×10 μm). The average value of the area proportions (%) of thepores in the five unit areas was taken as the porosity (vol. %). Thelargest pore size in the five unit areas (10 μm×10 μm) was taken as themaximum pore size. The average value of the pore sizes in the five unitareas was taken as the average pore size. The average value of thenumbers of the pores in the five unit areas was taken as the number ofpores. These measurement results are shown in Table 2 presented below.

TABLE 2 Average Number of Crystal Grain Average Maximum Pores per UnitStructure of Size Porosity Pore Size Pore Size Area Connected Sample No.(μm) (vol %) (μm) (μm) (pieces) Pores Example 1 2.2 20 0.8 3 22 ObservedExample 2 2.2 28 1 3.3 35 Observed Example 3 2.2 39 1.1 3.5 58 ObservedComparative 2.2 2 0.7 2.5 11 None Example 1 Comparative 2.2 66 2.8 8.2115 Observed Example 2 Example 4 2.2 20 1 2.8 30 Observed Example 5 2.229 1.8 3.4 48 Observed Example 6 2.2 40 1.9 3.7 59 Observed Comparative2.2 2 0.7 2.8 9 None Example 3 Comparative 2.2 56 2.7 7.9 103 ObservedExample 4

In the rare earth cold accumulating material particles according to eachof Examples, the respective parameters were within preferable ranges. Onthe contrary, Comparative Example 1 was low in porosity. ComparativeExample 2 was high in porosity.

Next, the refrigerating capacity was measured by using a refrigeratorhaving the rare earth cold accumulating material particles. As therefrigerator, a 4K pulse type refrigerator was adopted. In therefrigerator, a Cu mesh cold accumulating material was filled in thefirst cooling stage; in the second cooling stage, a group of lead coldaccumulating material particles was filled in the first group of coldaccumulating material particles, a group of HoCu₂ cold accumulatingmaterial particles was filled in the second group of cold accumulatingmaterial particles, and a group of rare earth cold accumulating materialparticles was filled in the third group of cold accumulating materialparticles. In the second cooling stage, Cu meshes as metal meshes wereused to partition the filling space.

When a cold accumulating material is filled in the second cooling stage,the cold accumulating material was filled while vibration was applied tothe second cooling stage so as for the mutual gaps between the coldaccumulating material particles not to be expanded. The Cu meshes of thesecond cooling stage were pressed into the second cooling stage with astress of 4 MPa and then fixed. After the filling and fixing operations,the refrigerator was operated, and the refrigerating capacity waschecked after elapsed times of 1000 hours, 20000 hours and 30000 hours.The check results thus obtained are shown in Table 3 hereunder.

TABLE 3 Refrigerating Capacity (W) After 1000 After 20000 After 30000Sample No. hours hours hours Example 1 1.5 1.5 1.4 Example 2 1.5 1.5 1.4Example 3 1.5 1.5 1.4 Comparative 1.3 1.3 1.3 Example 1 Comparative 1.21.1 0.8 Example 2 Example 4 1.5 1.5 1.4 Example 5 1.5 1.5 1.4 Example 61.5 1.5 1.4 Comparative 1.3 1.3 1.3 Example 3 Comparative 1.1 1 0.8Example 4

As can be seen from the results shown in Table 3, the refrigeratorsaccording to present Examples are effectively suppressed in thedegradation of the refrigerating capacity. The rare earth coldaccumulating material particles were taken out from each of therefrigerators after the 30000-hour operation, the change of the particleshapes was examined, and the groups of the rare earth cold accumulatingmaterial particles according to Examples were found to be free fromdestroyed particles.

On the other hand, as shown in Comparative Examples 2 and 4, the rareearth cold accumulating material particles having a porosity exceeding50 vol. % were low in strength, and were verified to have destroyedparticles. In addition, as shown in Comparative Examples 1 and 3, thecold accumulating material particles having a low porosity did not allowthe He gas to enter the interior of the particles, and accordinglydegraded the refrigerating capacity.

Thus, the refrigerators according to Examples were revealed to bedrastically improved in the long-term reliability. Accordingly,long-term reliabilities of various devices such as superconductingmagnets, inspection devices and cryopumps mounting such refrigeratorscan be drastically improved.

In the preceding description, several embodiments of the presentinvention are presented; however, these embodiments are presented asexamples, but have no intention to limit the scope of the invention.These novel embodiments can be implemented in various forms, and withina range not deviating from the gist of the invention, various omissions,replacements and alterations can be performed. These embodiments and themodified examples thereof are included in the scope or the gist of thepresent invention, and at the same time, included in the inventiondescribed in the scope of the claims and the equivalent scope thereof.The foregoing embodiments can be implemented as mutually combined.

REFERENCE SIGNS LIST

-   1 rare earth cold accumulating material particle-   2 rare earth compound (rare earth oxide or rare earth oxysulfide)    crystal grain-   3 pore-   4 structure of connected pores-   5 cold accumulating vessel-   6 metal mesh-   1-1 first group of cold accumulating material particles-   1-2 second group of cold accumulating material particles-   1-3 third group of cold accumulating material particles

1. A method for producing a rare earth cold accumulating materialparticle comprising the steps of: preparing a rare earth compound powderto be a starting material for the rare earth cold accumulating materialparticle; molding the rare earth compound powder as the startingmaterial thereby to obtain a spherically molded body; and sintering thespherically molded body at a temperature of 1200° C. or higher and 2000°C. or lower, thereby to produce the spherical rare earth coldaccumulating material particle consisting essentially of: a rare earthoxide or a rare earth oxysulfide; wherein the rare earth coldaccumulating material particle is a sintered body; wherein an averagecrystal grain size of the sintered body is 0.5 to 5 μm; wherein aporosity of the sintered body is 10 to 50 vol %; wherein an average poresize of the sintered body is 0.3 to 3 μm, and wherein in an arbitrarycross section of the rare earth cold accumulating material particle, anumber of pores per a unit area of 10 μm×10 μm is 20 to
 70. 2. Themethod for producing a rare earth cold accumulating material particleaccording to claim 1, wherein the starting material is formed of agadolinium oxysulfide (Gd₂O₂S) or a gadolinium aluminum oxide (GdAlO₃).3. The method for producing a rare earth cold accumulating materialparticle according to claim 1, wherein the rare earth compound powder tobe used as the starting material has an average particle size of 0.3 to5 μm.
 4. The method for producing a rare earth cold accumulatingmaterial particle according to claim 1, wherein a sintering aid powderis added to the rare earth compound powder, and a content of thesintering aid powder is set to be 1 part by mass or more and 20 parts bymass or less in relation to 100 parts by mass of the rare earth compoundpowder.
 5. The method for producing a rare earth cold accumulatingmaterial particle according to claim 4, wherein when an average particlesize of the sintering aid powder is represented by A (μm), and anaverage particle size of the rare earth compound powder is representedby B (μm), B/A is regulated so as to fall within a range from 0.7 to1.3.
 6. The method for producing a rare earth cold accumulating materialparticle according to claim 1, wherein a resin binder is added to therare earth compound powder so as for the addition amount of the resinbinder to be 10 to 50 vol % in relation to 100 vol % of the total amountof the rare earth compound powder and the resin binder.
 7. The methodfor producing a rare earth cold accumulating material particle accordingto claim 1, wherein the molding step is performed in accordance with atleast one method of a tumbling granulation and a die molding.
 8. Themethod for producing a rare earth cold accumulating material particleaccording to claim 1, wherein an average particle size of thespherically molded body falls within a range from 100 to 500 μm.
 9. Themethod for producing a rare earth cold accumulating material particleaccording to claim 1, wherein a sintering temperature is set to be 1500°C. or higher.
 10. The method for producing a rare earth coldaccumulating material particle according to claim 1, wherein a sinteringtime is set to 1 hour or more and 48 hours or less.
 11. The method forproducing a rare earth cold accumulating material particle according toclaim 1, wherein a rare earth oxide sintered body is subjected to a heattreatment in an oxygen atmosphere.
 12. The method for producing a rareearth cold accumulating material particle according to claim 1, whereina rare earth oxysulfide sintered body is subjected to a heat treatmentin a sulfur atmosphere including a sulfur oxide.
 13. The method forproducing a rare earth cold accumulating material particle according toclaim 1, wherein a surface polishing process is performed to thespherical rare earth cold accumulating material particle.
 14. The methodfor producing a rare earth cold accumulating material particle accordingto claim 1, wherein a spherical sintered body of the rare earth coldaccumulating material particle is subjected to a shape classification.15. The method for producing a rare earth cold accumulating materialparticle according to claim 1, wherein the spherical sintered body issubjected to a shape classification so as to select spherical sinteredbody having an aspect ratio of 2 or less.
 16. The method for producing arare earth cold accumulating material particle according to claim 1,wherein the shape classification is performed in such a manner that whenL represents a perimeter length of a projection image of each of thecold accumulating material particles constituting a group of rare earthcold accumulating material particles composed of the rare earth coldaccumulating particles, and A represents the actual area of theprojection image, in the group of rare earth cold accumulating materialparticles, a proportion of the rare earth cold accumulating materialparticles having a shape factor R being represented by L²/4πA andexceeding 1.5 is 5% or less.